The therapeutic strategies for lysosomal diseases such as mucolipidosis type IV (MLIV) are very limited. MLIV is an inborn disease characterized by disturbances in late endosomal and lysosomal processes in eye, CNS, gastrointestinal tract and other organs. It is a devastating disorder that usually presents during the first year of life with blindness and cognitive impairment. It is caused by mutations in MCOLN1, the gene encoding mucolipin-1 (ML), which we have recently established to represent a Ca2+ -permeable cation channel that is transiently modulated by Ca2+. In addition, our work has shown that naturally occurring mutant ML channels are only weakly activated by increases in intracellular Ca2+ (Cai) and show other functional abnormalities. We have characterized fibroblasts from MLIV patients and found disturbed Ca2+ signaling and large lysosomes with altered cellular localization. Other significant phenotypic alterations in these cells include defective late endosome/lysosome (LEL) fusion and disturbance of lysosomal exocytosis. These are Ca2+ dependent processes related to signaling pathways regulating Ca2+ homeostasis and lipid metabolism. There is a continued need for effective therapies for MLIV and other lysosomal disorders. In recent studies on lysosomal storage disorders the accumulation of mucoid and lipid materials in lysosomes was reduced using activators of lysosomal enzymes and other catabolic processes. In preliminary studies we found that similar agents in combination with dietary and defined pharmacological agents stimulating signaling pathways involved in Ca2+ homeostasis and lipid metabolism reduced the lysosomal enlargements and stimulated LEL fusion as well as lysosomal exocytosis in cells derived from patients with MLIV. We were also able to rescue the same phenotypic alterations by gene replacement approaches. The overall goal of this study is to characterize these phenotypic alterations and to design new approaches to correct and rescue them using gene replacement and combinations of synergistically acting dietary and pharmacological agents. We will characterize the pathological alterations in lysosomal morphology (Aim 1), ionic and metabolic regulation (Aim 2), membrane vesicle fusion (Aim 3) and exocytosis (Aim 4) occurring in the cells of patients with MLIV. Based on our preliminary studies we will also establish the optimal conditions to rescue all of these cellular phenotypes by gene replacement. Due to some of the limitations of the gene replacement interventions in Aim 5 we also propose alternative approaches based on stimulation of compensatory factors that may also contribute to the correction and rescue of each of the cellular phenotypes that are associated with MLIV. In the long term, these studies should help to develop new treatment modalities as potentially emerging therapies not only for MLIV but also for different types of mucolipidoses and other lysosomal storage disorders.